Haidinger's brush

Haidinger's brush is a subtle visual phenomenon that allows some people to perceive the polarization of light. It is named after Austrian physicist Wilhelm Karl von Haidinger, who first described it in 1844. This phenomenon appears as a faint, yellowish hourglass or bow-tie shape at the center of the visual field when looking at a region of polarized light with a blue sky being the most common natural source. The ability to see Haidinger's brush is thought to be due to the dichroic properties of the macula, the central part of the retina responsible for high-resolution vision.

Mechanism

The exact mechanism behind Haidinger's brush is not fully understood, but it is believed to involve the macular pigment, which is dichroic and absorbs more light in one polarization direction than the other. This differential absorption is thought to be visible against a uniformly polarized background, such as the sky. The macular pigment consists mainly of zeaxanthin and lutein, which are yellow pigments that can filter out blue light. The orientation of the molecules in the macula is such that they can interact with polarized light, allowing the observer to perceive Haidinger's brush.

Observation

To observe Haidinger's brush, an individual may look at a clear, blue portion of the sky 90 degrees from the sun, where the light is highly polarized. The phenomenon can also be observed by looking at a white surface through a polarizing filter, such as a pair of polarized sunglasses, and rotating the filter. Haidinger's brush is usually more easily seen through one eye than both and can be enhanced by staring at a bright light source, like a computer screen, for several seconds before observing a polarized light source.

Clinical Significance

While Haidinger's brush is primarily of interest as a physiological curiosity, it has some clinical significance. The ability or inability to perceive this phenomenon can provide information about the health of the macula. Since the perception of Haidinger's brush relies on the integrity of the macular pigment, difficulties in seeing it may indicate macular problems, such as macular degeneration. However, the absence of Haidinger's brush perception alone is not a definitive indicator of macular health.

Research

Research into Haidinger's brush continues, with studies focusing on its potential applications in assessing macular health and visual function. Understanding how and why some people can see Haidinger's brush while others cannot may also provide insights into variations in human vision and perception.

See Also

• Polarization (waves)
• Visual perception
• Macular degeneration
• Retina